1. Field of the Invention
The present invention relates to a photographing module comprising an electrically controlled focusing mechanism. More particularly, the present invention relates to a photographing module that uses a voice coil motor (VCM) structure as an electrically controlled focusing mechanism.
2. Description of the Related Art
There is a need to install an actuator for many electronic devices, such as a mobile phone with vibration function, a magnetic head having an actuator that moves vertically, a camera equipped with an auto focus module, etc. For those devices, the most popular type of actuator is the voice coil motor (VCM). Since VCMs are able to achieve precise movement and have lower prices, they are well suited for use as microactuators for short-distance movement within electronic devices.
A short-stroke microactuator can be used in a photographing module as an electrically controlled focusing mechanism for the auto focusing function. Referring to FIG. 1, a sectional view depicting the structure of a prior art photographing module 1 with an electrically controlled focusing mechanism is shown. The photographing module 1 mainly comprises a supporting base 40, a lens carrier 12, a lens unit 10 and a VCM. The supporting base 40 contains an accommodating space for the lens carrier 12 to be disposed therein. The lens unit 10 consisting of one or more lens elements is housed within the lens carrier 12, and moreover, there is thread engagement between the lens unit 10 and the lens carrier 12. A magnet 20, yoke 22 and coil 24 constitute the aforementioned VCM that is disposed within the accommodating space between the lens carrier 12 and the supporting base 40. The photographing module 1 is equipped with an elastic unit to provide a better moving and restoring speed of the VCM. The elastic unit includes an upper elastic member 32 and a lower elastic member 34. The upper elastic member 32 is disposed above the lens carrier 12, while the lower elastic member 34 is disposed below the lens carrier 12.
FIG. 2 is a top view showing the structure of a prior art upper elastic member 32 or lower elastic member 34. The elastic member 200 is of a flat plate shape, comprising an outer peripheral area 210, an inner peripheral area 220 and a plurality of bridge regions 230 connecting the outer peripheral area 210 and inner peripheral area 220. Each bridge region 230 has one end connected to the outer peripheral area 210 and the other end connected to the inner peripheral area 220. The outer peripheral area 210 of the elastic member 200 is secured to the supporting base 40, and the inner peripheral area 220 is secured to the lens carrier 12. When a current with a predetermined polarity is passed through the coil 24, an electromagnetic moving force generated. Since the coil 24 is attached to the lens carrier 12 and the lower elastic member 34, when the coil 24 is driven by the magnetic force, the lens carrier 12 will be pushed accordingly and move to a predetermined position along the optical axis, so as to perform an auto focusing function. The yoke 22 is used for enclosing the magnetic field lines between the coil 24 and the magnet 20. When a current with an opposite polarity is passed through the coil 24, the lens carrier 12 is driven to the original position.
Since the outer peripheral areas of both the upper elastic member 32 and the lower elastic member 34 are secured to the supporting base 40 while their inner peripheral areas are secured to the lens carrier 12, when the lens carrier 12 moves forward or backward relative to the supporting base 40 along the optical axis, stress is accumulated on the bridge regions 230 between the outer peripheral area and the inner peripheral area because those bridge regions 230 are elastically deformed. As shown in FIG. 2, the bridge regions 230 of the prior art elastic member 200 are in the shape of connected concentric arc segments; that is, a plurality of arcs of the same or similar shape which are connected to each other. When the elastic member is stressed as a result of the lens unit 10 being screwed into the lens carrier 12, or from a pulling force generated as the lens unit is shifted along the optical axis, or when the lens unit 10 is tilted, the resulting stresses cannot be absorbed by the arc-shaped portions of the bridge regions 230 because they all retain a similar shape. Consequently, stress is concentrated on the connecting joints between the bridge regions 230 and the outer/inner peripheral areas 210, 220. Accordingly, both the inner and outer peripheral areas 210, 220 will suffer too much internal vertical stress, thus over time it results in elastic fatigue of the connecting joints and thereby reducing the life span of the elastic member 200.